1. Field of the Invention
This invention pertains to the field of radio frequency identification (RFID) readers of the type used in access control systems for reading electronically coded key tags and key cards.
2. State of the Prior Art
RFID reader/writer units may be wall mounted in access control applications where a RFID transponder tag is waved near the reader by persons requesting access. One of the key elements of an RFID (Radio Frequency Identification Device) reader/writer is the antenna. The antenna can be a metal coil or a dipole, depending on the application and other particulars of the system. Radio frequency power is provided to the antenna by appropriate electronic circuits of the reader in a modulated form (the carrier frequency), which is then radiated by the antenna to any proximate RF transponder tag. Radiated power from the transmitting antenna is absorbed by the antenna of the transponder tag where it powers up the electronic chip of the transponder. The chip of the transponder then returns a coded signal detectable by the RFID reader unit. The carrier frequency is usually used as a clock signal for the transponder chip.
The reader communicates with the transponder by modulating the carrier frequency in a patterned or coded manner (usually pulse position modulation). The transponder communicates with the reader/writer using similar methods usually encompassing loading of the carrier signal by shunting the transponder antenna to effect a corresponding variation in the reader/writer antenna. This is usually accomplished using ASK (amplitude shift key), FSK (frequency shift key), PSK (phase shift key) or various other techniques using one or more sub frequencies.
The nature of RF energy in this type of application precludes it from penetrating an electrically conductive metal housing. Therefore, most RFID reader/writer units are designed such that the antenna is covered or enclosed in a non conducting material such as polycarbonate, epoxy or ABS. While this design allows the antenna to send and receive, the relative weakness of these materials leaves the antenna (and anything packaged with it) vulnerable to vandalism and tampering. In the past, installation and maintenance of RFID reader/writers of this type in high risk areas has been problematic.
Others have constructed reader/writers that are packaged with the antenna behind a thin metal plate with one or more narrow slits or holes in a metal plate covering the front of the reader unit. The slit(s) or hole(s) allow the radio-frequency field to propagate beyond the plate from the antenna inside the reader enclosure. However, the metal plate in these designs has thickness limitations, and as a result the added protection is minimal.
An example of such prior art efforts is shown in PCT Publication WO 01/50423 A1 entitled “Electronic Key Reader”. In this publication the antenna is a loop mounted behind a front metal plate of the RFID tag reader housing. One or more slots in the front metal plate allow propagation of the radio frequency field emitted by the internal antenna. However, the antenna loop is an element distinct from the metal plate or housing.
As previously explained, the RFID tags, used as identification tokens or keys to actuate RFID readers, generally consist of a radio frequency transponder module connected to a radio frequency antenna. The antenna may be a wire coil or may consist of one or more printed circuit loops. Typically, the transponder module and antenna are encapsulated in plastic for durability, such as in a plastic card. Existing plastic encapsulated RFID tags, while durable for use in operating doorways, elevators and garage gates in normal office, industrial and residential applications, are not sufficiently rugged for certain applications where such tags may be subject to a much greater degree of abuse, impact and mechanical wear, for example, in military applications. A need exists for more robust RFID tags better suited for rugged environments.